Multiple instances of one object/mesh
You have 2 solutions:

Linked Duplications: altD allows you to create a linked duplication, meaning if you modify one of the instances it will change all the others as they share the exact same mesh data (while a simple shiftD creates a non-linked duplication). To unlink a copy, select the object and Object &gt; Relations &gt; Make Single User &gt; Object &amp; Data (create a Q shortcut if you plan to use Make Single User a lot).

Collection Instances (former Group Instances): Place your cursor on the center of the grid with shiftC (the position of the object to the center of the scene will determinate the collection center). Create your object(s). Create a collection with your object(s) with M (before 2.8, CtrlG). Now with a shiftA &gt; Collection Instance you can create an instance of the collection you've created (shiftA &gt; Group Instance before 2.8). You can't change the instance, if you want to do it you need to edit the original object(s). If you want to make the instances &quot;real&quot; press CtrlA &gt; Make Instances Real (before 2.8, press shiftctrlA). It is still linked though, if you want to unlink it, go in the header menu &gt; Object &gt; Relations &gt; Make Single User &gt; Object &amp; Data (before 2.8 press U). As you can see, the instance is followed by an empty. The empty is placed where the grid center is relatively to the original object(s). If you want to change the position of the empty, select the original object(s) and move them or place the cursor where you want the empty to be, go in the Properties panel &gt; Object &gt; Collections, click on the down arrow and choose Set Offset From Cursor (before 2.8, Properties panel &gt; Object &gt; Groups).


As you can see, linked duplication is much easier to use, and it doesn't weigh more than group instance, so most of the time I guess you should choose this solution. But as said by BatFINGER below, group instance can be very useful sometimes (you can mix several types of objects, modifiers or constraints will affect every object...).